1. Technical Field
The present invention relates to a mount structure of an intake air flow control valve device. In particular, the present invention relates to a mount structure for mounting, on a cylinder head, an intake air flow control valve device that is disposed in an intake manifold of an engine and controls an intake air flow formed in a combustion chamber.
2. Related Art
An intake air flow control valve device that is disposed in a resin intake manifold and controls an intake air flow formed in a combustion chamber is proposed, as disclosed in Japanese Unexamined Patent Application Publication (JP-A) No. 2007-303327, for example.
The intake air flow control valve device is applied to a four-cylinder engine, and includes a resin intake manifold 101 and valve units 104, as illustrated in an exploded perspective view of FIG. 7. In the intake manifold 101, four intake air passages 102 are formed by separation walls 101a, and each of the valve units 104 is disposed in each of the intake air passages 102.
The valve unit 104 includes a frame shaped housing 105, an intake air flow control valve 106, and a valve shaft 109. The plate shaped intake air flow control valve 106 has bosses protruding to bath sides, and each of the bosses is rotatably supported by a supporting hole of the housing 105 via a bearing. The valve shaft 109 penetrates a separation wall through-hole 101b of the intake manifold 101 and holes formed at the bosses of the intake air flow control valves 106. Thereby, the intake air passages 102 are opened and closed by synchronous rotation of the intake air flow control valves 106 in association with rotation of the valve shaft 109.
Further, in an intake air flow control valve device disposed in another intake manifold, a flange 114 is provided at an end of a housing 112 including intake air passages 113A and 113B, as illustrated in FIG. 8, which is a cross-sectional view of principal parts. The intake air passages 113A and 113B are adjacent to each other and communicated by a shaft penetrating unit 116 that has a shaft hole 116a. 
A mount surface of the flange 114 formed at the end of the housing 112 is provided with annular gaskets 115a and 115b along respective opening outer peripheries of the intake air passages 113A and 113B.
A valve shaft 118 penetrates through the intake air passages 113A and 113B, and the shaft hole 116a. The distal end of the valve shaft 118 is rotatably supported, via a bush 119, by a supporting hole 112a formed at the outer end of the intake air passage 113B of the housing 112. The base end of the valve shaft 118 is coupled with an actuator 120, such as an electric motor, provided outside the intake air passage 113A of the housing 112. Plate shaped intake air flow control valves 117A and 117B, which are disposed in the intake air passages 113A and 113B, respectively, are provided on the valve shaft 118. Thereby, the intake air passages 113A and 113B are opened and closed by synchronous rotation of the intake air flow control valves 117A and 117B in association with rotation of the valve shaft 118 by the actuator 120.
In an intake air flow control valve device 111 thus configured, the flange 114 is bolted to a mount surface 151 of a cylinder block 150, where intake air ports 152a and 152b are opened via gaskets 115a and 115b. 
According to JPA No. 2007-303327, the bosses, which is provided at the both sides of the respective intake air flow control valves 106, are rotatably supported by the housings 105 via bearings. However, the resin intake manifold and the housing 105 are not uniform in manufacturing shape and dimensional accuracy, and have low rigidity, compared with the conventional intake manifolds and housings made of metal, such as aluminum. Thus, deformation may be caused by environmental changes, such as increases and decreases in temperature by use. The deformation of the intake manifold and the housing 105 may hinder smooth operation due to deterioration in concentricity between the bosses of the intake air flow control valves 106 and the bearings.
In the intake air flow control valve device 111 illustrated in FIG. 8, deformation of the housing 112 caused by environmental changes, such as increases and decreases in temperature may also occur, since the resin intake manifold and the housing 112 are not uniform in manufacturing shape and dimensional accuracy, the resin housing 112 and the metal valve shaft 118 have different coefficients of thermal expansion respectively, the actuator 120 is disposed at the outer end at one intake air passage 113A side of the housing 112, and the bush 119 to pivotally support the distal end of the valve shaft 118 is disposed at the outer end at the other intake air passage 113B side of the housing 112. For example, as indicated by a virtual line 112b, the housing 112 may be deformed into a curved shape in a direction in which the end at the intake air passage 113B side, where the bush 119 is disposed, move away from the mount surface 151 of the cylinder head 150, with respect to the end at the intake air passage 113A side, where the actuator 120 is provided.
In this deformation, the displacement amount by which the bush 119 pivotally supporting the distal end of the valve shaft 118 moves away from the cylinder head 150 becomes large, and thus, the tilt of the shaft hole 116a of the shaft penetrating unit 116 may become larger than the tilt of the valve shaft 118. Accordingly, the concentricity between the valve shaft 118 and the shaft hole 116a is deteriorated, and thus, the valve shaft 118 and an inner peripheral surface of the shaft hole 116a come into contact with each other. As a result, operating performance may be possibly deteriorated.
If the shaft hole 116a having a large diameter is formed so as to avoid the contact between the valve shaft 118 and the shaft hole 116a of the shaft penetrating unit 116, a large gap is formed between the inner peripheral surface of the shaft hole 116a and the valve shaft 118. The intake air flowing through the intake air passage 113A and the intake air flowing through the intake air passage 113B are communicated and interfered with each other through the gap, thereby generating turbulence in the intake air passages 113A and 113B. As a result, deterioration in intake characteristic occurs since an intake air flow in a combustion chamber is not smoothly generated, and thus, combustion efficiency of the engine lowers, resulting in lowering output.